The invention relates to apparatus for determining the color composition of printing light in photographic enlarging apparatus.
Prior forms of such measuring apparatus have a measuring probe which contains at least one photo-electric transducer, and which is introduced into the printing light ray path immediately below the objective of the enlarging apparatus during the measuring operation. The measuring probe is connected by way of an electrical lead to an evaluating apparatus containing a measuring circuit and an indicator unit. One form of measuring apparatus is miniaturised to such an extent that the measuring probe and evaluating apparatus form one unit which can be pivoted into the path of rays immediately below the objective of the enlarging apparatus for the measuring operation.
The introduction of a measuring probe or a measuring apparatus into the path of rays leads to rather complicated manipulation, since the objective which is covered over by the measuring probe during measuring must be free during the printing operation in order that the printing light can reach the projection surface. This disadvantage is particularly noticeable when the filter determination for which the measuring apparatus is used takes place before the printing operation for each individual original, since frequent introduction and removal of the measuring probe is then necessary.
The light which passes through the objective is influenced not only by the original which is situated in the enlarging apparatus during the measuring operation, but generally also by a variable aperture diaphragm. If each color component of the printing light is measured on its own rather than being compared to another color component, care must be taken to insure that the same diaphragm value is set during each measurement. Measurements are normally carried out with the diaphragm fully open, since the maximum measuring light is then available. Measurements are normally also carried out with a fully open diaphragm in the case of measuring apparatuses with which the relationship between individual color components is measured, since it may be that only in this state is there sufficient light available for the photo-electric transducer arranged in the measuring probe. It therefore becomes necessary for the operator of the enlarging apparatus to adjust the objective diaphragm before and after the measuring operation, since printing is not carried out with a fully open diaphragm in most cases.
Enlarging apparatuses are known which allow prints to be made in daylight or with lighting similar to daylight, being provided for this purpose with a lightproof closed screening arrangement between the objective and the projection surface. Apparatuses of this type suffer from the disadvantage that the introduction of a measuring probe below the objective is very complicated, since the screening arrangement must be removed for this purpose and fitted again for the printing operation.